There has been known a conventional ultrasonic testing method that transmits ultrasonic waves from an array probe having transduces linearly arranged through a flange-side rim face of a wheel such as a railway wheel so as to detect a flaw in a rim portion of the wheel (in the rim portion, one of two side faces vertical to a shaft of the wheel, on which a flange is formed, is referred to as a “flange-side rim face” in the present specification). In this method, ultrasonic testing is carried out with the array probe opposite to the flange-side rim face while rotating the wheel in the circumferential direction. Generally, when flaw detection is carried out while rotating the wheel, an angle between the transducer alignment direction (also referred to as a “longitudinal direction”, hereinafter) of the array probe and the radial direction of the wheel is set to be 0° when viewed in the axial direction so as to secure a greater area of a region where the ultrasonic waves enter the surface of the wheel (also referred to as a “incident region”, hereinafter).
Unfortunately, in this ultrasonic testing, the array probe receives an echo reflected from a shape having surface texture roughness of the flange-side rim face, etc. (hereinafter referred to as a “shape echo of the surface texture”); and therefore, if a flaw exists in the vicinity of the flange-side rim face, and if intensity of the flaw echo of this flaw is equal to or less than intensity of the shape echo of the surface texture, it is difficult to detect this flaw. Hence, it has been desired to reduce the intensity of the shape echo of the surface texture of the flange-side rim face.
In the ultrasonic testing using the array probe, there has been known a method that determines an incidence angle of ultrasonic waves to an object to be detected so as to prevent shape echoes from a region other than a flaw detection region from returning to the array probe (see Patent Literature 1, for example). Unfortunately, this method cannot reduce intensity of a shape echo returning from the surface texture of the object to be detected opposite to the transducer face of the array probe.